The starter motor's commutator is a ring of copper segments on the armature; the carbon brushes press on it to feed current into the rotating windings, and smooth, spark-free current transfer needs a clean, round, correctly insulated surface.

When a commutator is damaged its face becomes burnt, rough or out-of-round, and the mica insulation between segments may stand proud, so the brushes bounce, arc and lose steady electrical contact.

The correct repair is to recondition the commutator surface: skim it true on a lathe, undercut the mica between the segments and clean the copper so a properly grounded, evenly-spaced conducting face is re-established for the brushes to ride on.

This reconditioning is what “regrounding” refers to — restoring the true, insulated copper surface so current transfer is once again smooth.

Tightening only firms up loose fasteners and does nothing to the worn conducting face, while lubricating a commutator is actually harmful because oil insulates and fouls the brush contact.

Rewinding replaces the armature coils, which addresses winding faults rather than the damaged commutator surface itself, so it is not the direct remedy here.

Hence, the answer is Regrounding.